Bug 862870. Implement splice batching with forced flushes if batching too much. r=asuth
Bug 862870 - [email] Implement slice batching to reduce/minimize UI reflows, improve responsiveness, sync time. r=asuth, review fixes.

Minor naming nits and some test cleanups/improvements.  I also added some docs
to the test helpers in the process of adding new helpers.
Merge pull request #280 from asutherland/batch-slice-review

Bug 862870 - [email] Implement slice batching to reduce/minimize UI reflows, improve responsiveness, sync time. r=asuth